1. Field of the Invention
The present invention relates generally to power measurement circuits and systems, and more particularly to a power measurement system that use a complex wavelet filter to provide input for power measurement and system calibration.
2. Background of the Invention
Power measurements performed by integrated circuits (ICs), such as those used in motor control and power supply systems typically measure voltage and current delivered to a load using analog-to-digital converters (ADCs) and then compute a measured power level by multiplying the voltage and current measurements to obtain a power measurement. Systems such as those disclosed in U.S. Pat. No. 5,521,482 typically assume that the measured power is a real power value, or for A/C power measurements, use a power factor to adjust the power calculated from the voltage and current measurements to obtain the real power. Such power computations are accurate for a stable fixed-frequency sine wave. For DC power measurements, the power factor correction is not needed.
However, making accurate A/C power measurements when the frequency and/or amplitude varies rapidly, such as in some A/C motor control applications, and when jitter is present is very difficult and requires extensive computations in order to preserve accuracy of the measurements. Further, measurement of complex power including both real and reactive power metrics is desirable for total modeling of the load conditions which is useful in monitoring and controlling power delivery in applications such as motor control.
Techniques for extracting relative phase and amplitude from voltage and current measurements to determine line power characteristics typically require filtering due to the presence of harmonics and noise on the measurement inputs and since the voltage and current measurements are filtered separately, calibration between the voltage and current channels is typically required to provide accurate input values of line voltage and current for subsequent power calculations. Calibration typically requires additional circuitry and calibration time, and therefore it would be desirable to avoid the necessity of calibrating the channels.
To perform power measurements on a line-cycle basis from sampled data, it is typically necessary to determine the relationship between the line frequency and the sampling frequency, both of which may vary from a specified value, with line frequency typically varying by a significantly larger factor. Typical power measurement circuits determine the line frequency using a zero-crossing detector in conjunction with higher-frequency counters. However, such measurements are affected by the presence of line harmonics and jitter. Therefore, it would be desirable to measure line frequency more accurately.
Therefore, it would be desirable to provide a system and method for measuring voltage/current phase and amplitude for performing line power measurements and calibration of the system. It would further be desirable to provide such a system and method that can determine line frequency accurately.